Electrolytes for lithium- and lithium-ion batteries typically require non-aqueous electrolytes with low water content in order to obtain acceptable calendar and cycle life. Commercial electrolytes for lithium-ion batteries typically specify water contents of no more than 500 parts per million (ppm) by weight, typically no more than 400 ppm, often no more than 300 ppm, still more often no more than 200 ppm, yet still more often no more than 100 ppm, and most often below 50 ppm by weight. These electrolytes typically comprise one or more non-aqueous or aprotic solvents and one or more lithium salts, such as lithium hexafluorophosphate (LiPF6). A common electrolyte production method includes removing the water from a solvent mixture and then mixing in a lithium salt powder. It is difficult and expensive to remove water from the electrolyte once the lithium salt is combined with the solvent, so the lithium salt added to the electrolyte must have very low water content, typically less than 100 ppm.
Recently, battery manufacturers have begun to use lithium bis(fluorosulfonyl)-imide (“LiFSI”) as an alternative to the more commonly used LiPF6 salt in electrolytes for lithium- and lithium-ion batteries. LiFSI salts provide higher conductivity solutions, and are more stable against hydrolysis, which improves battery lifetime and performance at elevated temperatures. NaFSI salts have also been reported for sodium-ion batteries, and KFSI is a candidate for use in both potassium and sodium-ion batteries.
Production of dry, free-flowing alkali metal salts of bis(fluorosulfonyl)imide with low water content is technically challenging. LiFSI is deliquescent (i.e., it can absorb enough water vapor from the air to form a liquid solution), and therefore all handling is typically performed in closed systems or dry rooms with very low humidity. In addition, LiFSI/water mixtures are not stable at high temperatures, for example, 50% LiFSI/water mixtures can decompose violently above 120° C. More importantly, concentrated LiFSI/water mixtures can form complexes with extremely low vapor pressure. Hydrolysis of the LiFSI salt proceeds rapidly at temperatures above 60° C., so it is not practical to heat water-containing LiFSI powder or solutions to higher temperatures in order to speed the water removal. This makes final removal of water from LiFSI powders complex and expensive.
Prior art methods for drying LiFSI salts include vacuum drying, anti-solvent precipitation, and water vapor stripping with dry gas. As an example, U.S. Pat. No. 9,079,780, issued to Sato (the “Sato '780 Patent”), describes a method for producing LiFSI using a combination of dry gas and vacuum drying in a short path distillation apparatus. U.S. patent application publication number 2013/0323155A1, filed by Tsubokura, describes production of dry LiFSI by precipitating the powder from a concentrated solution using an anti-solvent such as methylene chloride. These prior art methods all suffer from high energy use, inefficient solvent use, low yield (typically <80%, often <60%) or very long drying times. State-of-the-art methods for preparing lithium salts for use in electrolytes generally involve crystallization and drying steps to produce free-flowing powders. Electrolyte preparation requires handling these powders without introducing water, which increases the cost and complexity of electrolytes.
Attempts to remove water from LiFSI have proven extraordinarily difficult using conventional methods. Simply removing water as a vapor by evacuating LiFSI/water mixtures at temperatures <40° C. are not practical due to the very long time and high energy use required. Example 3, from Sato's '930 patent describes a 7-day drying time in a shelf-type vacuum dryer to obtain a powder. As LiFSI becomes more concentrated in water, the vapor pressure of the water falls far below that predicted by the ideal case of Raoult's law. This effect was demonstrated by He et al. (U.S. Pat. No. 9,268,831) for other solvents. While heating the concentrated solution would theoretically raise the solvent vapor pressure, LiFSI will quickly consume water by hydrolysis at elevated temperatures and generate acids of fluoride, sulfate, and sulfamate, which are undesirable in LiFSI electrolytes.
Most prior art processes for producing dry lithium salts require the use of volatile organic solvents. If these solvents are not components of the final electrolyte, they must be removed from the salt before final electrolyte blending. Removal of these solvents is time consuming, expensive, and presents both safety concerns from the handling of flammable solvents and environmental hazards due to potential release of volatile organic compounds into the environment.
Therefore, there is a clear unmet need for simplified methods of delivering lithium salts for electrolytes that avoids the need to handle solids and provides an electrolyte of high purity and low water content.